Leo-Wang Chen

Study on shape-memory behavior of polyether-based polyurethanes. I. Influence of the hard-segment content

Shape-memory polyurethanes (PUs) were synthesized by 4,4′-diphenylmethane diisocyanate (MDI), 1,4-butanediol (BD), and poly(tetramethyl oxide)glycol (PTMO). The morphology of PUs was studied using DSC, DMA, and TEM. The results indicated that the PUs would show different morphology by changing the mol ratio of MDI and BD (hard segment). The specimens would show the shape-memory behavior that was fixing and recovering the deformation at different operating temperature ranges. These results demonstrated that the shape-memory behavior of the PUs would be affected by the morphology and the modulus ratio that was defined as in DMA analysis. The amount of the hard-segment-rich phase would affect the ratio of recovery, that is, the low content would lead to the recovery of the deformed specimen being incomplete. The recovering rate would be influenced by the modulus ratio and the size of the dispersed phase in the micromorphology. On the other hand, the shape-memory behavior of PUs could be enhanced by the training process.

Study on shape-memory behavior of polyether-based polyurethanes. II. Influence of soft-segment molecular weight

Polyurethanes (PUs) of a suitable molar ratio of monomers were found to have shape-memory behavior. In this study, four series of PUs were synthesized by 4,4′-diphenylmethane diisocyanate (MDI), 1,4-butanediol (BD), and various molecular weights of poly(tetramethylene oxide)glycol (PTMO) to study the influence of the soft segment (PTMO) on the shape-memory behavior of PUs. The investigation on thermal and dynamic mechanical properties was performed using DSC and DMA; then, the morphology of the PUs was directly observed by TEM. At the range of the individual glass transition, a similar recovering behavior was found from the deformed specimen that contained the same composition but different molecular weights of PTMO. However, the phase separation between the soft and the hard segments of the PUs would influence their shape-memory behavior. On the other hand, a large number of the dispersed phase of the PUs would delay the recovery rate of the deformed specimens using a high molecular weight PTMO.

Shape-memorized crosslinked ester-type polyurethane and its mechanical viscoelastic model

A series of shape-memorized crosslinked ester-type polyurethanes (PUs), based on different compositions of 4,4′-diphenyl methane diisocyanate (MDI), poly(butylene adipate) glycol (PBAG) with different molecular weight (MW) and trimethylol propane (TMP), were synthesized. The morphology of samples was investigated by using DSC, WAXD, and dynamic mechanical analysis (DMA). It was found that the morphology of the soft segment, which was PBAG with a different MW, was in an amorphous state and no crystalline domain was found. By increasing the crosslinked density (varying the content of TMP) or decreasing the length of the soft segment (MW of PBAG), the glass transition temperature of studied samples increased. But the range of transition broadened and the modulus ratio E′−20°C)/E′+20°C) also decreased. The shape-memory behavior was studied by the bending test method adopted from the shape-memory alloy. The sample with high Tg showed not only a high recovered temperature (Tr) but also a high recovered rate (Vr) with a high modulus ratio. By introducing the chemical crosslinked structure, the deformed samples completely recovered their original shape and rendered shape-memory behavior under the complex deformation. The shape-memorized crosslinked ester-type PUs can be applied at different operating temperatures. A mechanical viscoelastic model is discussed for the shape-memory behavior of PUs, and the modified Bonarts viscoelastic model properly describes the mechanism of the shape memory of PUs.

Evaluation of asymmetric poly(vinyl alcohol) membranes for use in artificial islets

Islets of Langerhans surrounded by a semipermeable membrane to prevent the host immunosystem is a potential way to treat type I diabetes mellitus. In this study, a series of poly (vinyl alcohol) membranes were formed by adding polyethylene glycols to create pores in the skin layer. The permeability study showed the skin layer structure had an influence on the diffusion of low molecular weight glucose, vitamin B12 and insulin. The mass transfer coefficient was improved from 1.04 x 10(-4) to 2.16 x 10(-4) cm/ sec for glucose, from 2.84 x 10(-5) to 8.36 x 10(-5) cm/sec for vitamin B12 and from 1.45 x 10(-6) to 4.15 x 10(-6) cm/sec for insulin, whereas the passage of immunoglobulin G was completely prevented, indicating that these membranes could be effective in protecting islets from immunorejection. Thus such a membrane is an alternative potential material for artificial islets. In addition, we examined the insulin secretory response of islets separated by a poly(vinyl alcohol) membrane. We found that the insulin-secretion rate is relatively rapid compared to the permeation rate of insulin; thus, the process of the artificial islets is insulin-diffusion-controlled.

The curing reaction and physical properties of DGEBA/DETA epoxy resin blended with propyl ester phosphazene

The influences of different amounts of propyl ester phosphazene (FR) on the curing kinetics and physical properties of diglycidyl ether of bisphenol A (DGEBA) epoxy prepolymer cured with diethylenetriamine (DETA) were investigated with DSC, SEM, DMA, and tensile testing. The results revealed that FR could be a catalyst or a diluent depending on the FR content. In addition, the blending systems were partially miscible. The tensile strength and modulus of blends decreased with increasing amounts of FR, but the elongation increased with increasing FR.

The glycolysis of poly(ethylene terephthalate)

The experiments were carried out in excess ethylene glycol (EG) and poly(ethylene terephthalate) (PET) oligomer in the presence of zinc acetate as catalyst at 190°C. It was found that during the glycolysis reaction, the monomer and dimer were contained in the liquid phase. After 2 h of the glycolysis reaction, an equilibrium state between the monomer and dimer was seen to approach. The equilibrium value of 0.51 between the monomer and dimer obtained at 190°C was in good agreement with previously published values. It was found that the catalyst concentration facilitated equilibrium and increased the glycolysis rate. A reduced amount of EG in the initial feed resulted in a decrease in the glycolysis rate.

Aryl phosphinate anhydride curing for flame retardant epoxy networks

An aryl phosphinate anhydride (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide)-methyl succinic anhydride (DMSA), with a pendent phosphorus group was synthesized and used as a reactive type flame retardant. The structure of DMSA was confirmed by FTIR, mass, elemental analysis, 1H and 31P NMR spectroscopies. The aryl phosphinate anhydride was successfully used to cure some commercial epoxides, such as bisphenol A diglycidyl ether (DGEBA) and epoxy-novolac. The epoxy networks obtained were characterized by dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and the limiting oxygen index (LOI) test. Epoxy resins cured with DMSA exhibit higher LOI values and char yields compared with those cured by commercial anhydrides, such as phthalic anhydride (PA) and hexahydrophthalic anhydride (HHPA). Furthermore, the LOI value and the char yield of the epoxy-resin composites increase with increasing phosphorus content. The flame retardancies of the epoxy-resin composites were significantly improved by using aryl phosphinate anhydride.

The effect of morphology variety of EVAL membranes on the behavior of myoblasts in vitro

Not only the surface morphology but also the surface chemistry can be changed during the fabrication of biomaterials. Therefore, the result of a biocompatibility test of one material may alter to a great extent, dependent on the fabrication process. In this paper, the in vitro interaction of myoblasts and EVAL membranes with different surface properties was investigated. It was observed that moderate contact angle and porous structure are favourable for the cell adhesion and growth. However, cell adhesion and growth were decreased on a porous structure with particulate morphology and higher contact angle.

Thermal degradation behavior and flammability of polyurethanes blended with poly(bispropoxyphosphazene)

Polyurethanes containing different amount of flame retardant, poly(bispropoxyphosphazene), were synthesized by a two-step polymerization. The thermal degradation behaviors of these polyurethanes were then studied by the thermal gravimetric analysis (TGA), TGA coupled with Fourier transform infrared analysis and elemental analysis. A limiting oxygen index was used to evaluate the flammability of these polyurethanes. For these modified polyurethanes under nitrogen, a two-stage thermal degradation behavior was observed. The first stage was caused by the degradation of hard segments, whereas the soft segments were responsible for the second-stage degradation. The thermal degradation activation energies were calculated by using Ozawas method. It was found that the addition of flame retardant caused a decrease of the activation energy in the first stage, but an increase in the second stage, which was probably due to the formation of a thermal stable structure. As for the flame retardancy, the modified polyurethanes have a higher char yield at 550°C, and a higher limiting oxygen index than the neat polyurethane.

Effects of precipitation conditions on the membrane morphology and permeation characteristics

The permeability and permselectivity of asymmetric and particulate membranes towards glucose and proteins of various molecular sizes were studied. It was found that the skin layer of asymmetric membranes was permeable to glucose and insulin but effectively prevent the permeation of immunoglobulins. This result parallels our interest for the development of artificial pancreas. It was also found that skinless particulate membranes exhibited not only high permeation rates with respect to albumin and immunoglobulins but also good selectivity between these components. Thus, particulate membranes has the potential to be used in separating albumin from immunoglobulins for treating disorders related to immunoglobulin abnormalities.